Cavendish
The Cavendish balance consists of a light, rigid T-shaped member supported by a fine vertical fibre. Two small spheres of masses m are mounted at the ends of the horizontal portion of the T. A mirror M on the vertical reflects the beam of light on to a scale.
Cavendish balance
Two large spheres of masses M are brought near m as shown in the figure. The gravitational attraction between m and M turns the system through an angle measured by the reflected light beam.
The force per unit twist is calculated by preliminary experiments. Since F, m, M and the distance between m and M are known, G can be calculated.It was found that
G = 6.670 x 10-11 Nm2kg-2